302 



MAGNETIC METHOD 



[Chap. 8 



in the sample at a distance 2a from the needle is — t—^ ■ ni> where m is 



the pole strength of the needle (= M/l), y is the permeability of the speci- 

 men, and vo the permeability of air. The field of the induced pole is 

 pole strength divided by distance squared. Therefore, for a vertical 

 magnetometer : 



II — Vo m 



AZ = 



V-\- Vo 4a2 



(8-9o) 



Fig. 8-36. Astatic magnetometer for susceptibility determination (after 

 E. A. Johnson and W. F. Steiner). 



Since y = ^itk and yo = 1 (air), 



AZ = 





(8-96) 



The last equation holds for only a very thick specimen. If the thickness is 

 limited and equal to h, 



ir/cm 



AZ = 



2(a + hy' 



(8-9c) 



In practice, it is preferable to use eq. (8-96) and to correct for thickness 

 and cross section.® 



« J. Koenigsberger, Terr. Mag., 34(3), 209 (Sept., 1929). 



